The present invention relates to a method for the continuous preparation of monomeric or oligomeric alkoxysilanes by the esterification of chlorosilanes with alcohols, in the presence of water, if desired.
The reaction of chlorine bound to silicon with alcohols to produce alkoxysilanes has been known since the middle of the 19th century (cf., e.g., Ebelmann, Ann. chim. pharm. 52. 322 (1844), 57, 319 (1846); Friedel, Crafts, Ann. chem. Phys. 2 (4), 3 (1866)), and is used commercially in a number of processes.
The synthesis can be performed either in the gas phase or liquid phase. The known processes, however, differ from one another mainly in the manner of removal of the hydrogen chloride formed during the reaction. It is essential that the formed hydrogen chloride be removed in order to (1) assure a complete reaction; (2) obtain a neutral product; and (3) prevent undesirable secondary reactions, such as, for example, the formation of alkyl chlorides. It is desired that the hydrogen chloride content in the end product be less than 50 ppm and preferably less than 20 ppm.
The reaction of chlorosilanes with alcohols in the liquid phase in a vessel equipped with a stirrer is described, for example, in British Pat. No. 674,137. The reaction in a reaction tube (DE-OS No. 20 33 373) or in a packed column (DE-OS No. 26 43 074) is also known.
Likewise, a variety of methods have been described, in which the reaction is performed in heated packed columns or distillation columns (cf., e.g., A. Raskai, Chem. Techn. 9 (8), 463 (1957); DE-OS No. 20 61 189; DE-PS No. 24 27 085; DE-OS 28 06 036 and DE-OS No. 30 00 782).
The use of inert gas in sprinkler columns and distillation columns has been described (German Pat. No. 862,895; DDR Pat. No. 31 751) for the removal of hydrogen chloride.
The removal of hydrogen chloride can furthermore be accomplished by boiling (in the presence of an inert solvent, if desired), or by distilling the raw product (cf., e.g., DE-OSs Nos. 20 61 189; 38 01 618; 24 09 731; 27 44 726; 28 00 017 and 28 06 036).
All of these known procedures have at least the disadvantage that the reaction, and especially the freeing of the end product of hydrogen chloride, has to be performed in several process steps or by the constant use of adjuvants which do not participate in the reaction.
The problem therefore existed of finding a continuous process to permit the preparation of monomeric and especially of oligomeric alkoxysilanes in the desired purity, without the use of additional adjuvants or process steps.